Because Felrush and Vengeful Retreat can put you in bad positions often during high movement fights or fights that require grouping often.
If a Boss stands perfectly still and has a large hitbox, Momentum feels ok. But I'd rather be able to keep Fel Rush and Momentum as movement abilties than as DPS abilities.
I also just find other DPS variants of Havoc much more fun and faster paced. Momentum feels slow regardless of how much you are actually zipping around the place.